Paragraph UG-28 of the ASME Section VIII, Division 1 provides rules for reinforcement of the cone-to-cylinder junction based on line of support criteria. Code Case 2286, on the other hand, provides rules based on actual stresses versus allowable stresses. In this paper, the authors compare the results of the calculations for the reinforcement of the cone-to-cylinder junction for each method. It was found that, in some cases, the reinforcement method of UG-28 will produce non-conservative results.
As an industry consensus, API 934-A is an excellent recommended practice on the materials and fabrication requirements for Cr-Mo reactors. However, it is cautious and somewhat vague on the topic of Intermediate Stress Relief (ISR) versus Dehydrogenation Heat Treatment (DHT) for the different types of welds — which reflects the industry’s varying practices. For the advanced steels, API 934-A states that DHT should only be used with Purchaser approval, and that it should not be used on restrained welds such as nozzle welds. As a result, it is common for a DHT to be permitted on longitudinal and circumferential seams to achieve the cost and schedule savings, and ISR is used for nozzle welds. There are risks to the fabricator however, as the welds remain extremely brittle after DHT (the toughness is restored after postweld heat treatment {PWHT}, and at intermediate levels after ISR), and welding defects that are acceptable per ASME Code criterias can lead to brittle fractures during subsequent fabrication steps. The costs of the repairs and delays can then be very high, especially if the cracking is not detected until after PWHT. This paper shows the risks of acceptable defects causing brittle fractures by fracture mechanics calculations, and presents some case histories of cracking. The relative costs of ISR versus DHT, versus repairs before and after PWHT are also reviewed.
The 2007 ASME Section VIII, Division 2, Part 5 Design-By-Analysis provides requirements for vessels and components using analytical methods. The authors have spent a considerable amount of time in studying and applying the requirements of the design by analysis methods in Part 5. As a result of this process, the authors have concluded that the main factor for weight savings is the stress criteria used to calculate the thickness by the design-by-analysis method. An example is provided to demonstrate the methods presented.
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